Salmonellae are facultative intracellular pathogens which cause significant diseases in humans and animals. These organisms cause several disease syndromes including enteric (typhoid) fever, gastroenteritis, bacteremias, and focal infections. Typhoid fever is a severe systemic illness which is mostly a problem in the developing world and in travelers. Non-typhoidal Salmonella infections are increasing in the United States and are largely associated with contaminated food. Recently several outbreaks of S. enteritis associated with contaminated intact shell eggs have been a major problem in the United States. Salmonellae infections are more severe in infants, the elderly, and in immunosuppressed individuals. This is a particular problem for patients with AIDS, as such individuals develop severe and recurring infections. In fact recurrent Salmonella bacteremia is an AIDS defining opportunistic infection. Murine infection with S. typhimurium is an important model for studies of typhoid fever pathogenesis and vaccine development because the causative agent S. typhi only infects humans. This grant proposes to study this model system and several tissue culture models of infection, including one that models human gastroenteritis. The role of one set of bacterial virulence genes that are coordinately regulated is the focus of this grant. This system termed the PhoP/PhoQ regulon is composed of genes important to the pathogenesis of typhoid fever, and based upon an in vitro tissue culture model, gastroenteritis. A set of genes that are PhoP-activated (pag) are expressed within acidified macrophage phagosomes and promote organism survival within macrophages, which is an essential property necessary to cause typhoid fever. Another set of genes termed PhoP-repressed genes (prg) are essential to signaling eucarytotic cells to initiate cytoskeletal rearrangements that ultimately lead to organism internalization. This property is important to colonization of epithelia and is likely important to cross the intestinal mucosal barrier. prg also are involved in signaling at epithelial apical surfaces to stimulate inflammatory neutrophil transmigration across an intact monolayer. This inflammation likely contributes to diarrhea. This grant proposes to further define these virulence genes, to study in molecular detail their regulation and role in bacterial virulence.